The One Big Union is an idea originating in the late 19th and early 20th centuries amongst trade unionists to unite the interests of workers and offer solutions to all labour problems.

Unions initially organized as craft unions. Workers were organized by their skill: carpenters, plumbers, bricklayers, each into their respective unions. Capitalists could often divide craft unionists along these lines in demarcation disputes. As capitalist enterprises and state bureaucracies became more centralized and larger, some workers felt that their institutions needed to become similarly large. A simultaneous disenchantment with the perceived weakness of craft unions caused many unions to organize along industrial lines. The idea of the "one big union" is championed by anarchist syndicalists to organize effectively.

As envisioned by the Industrial Workers of the World (IWW), which for many years prior to 1919 had been associated with the concept, One Big Union was not just the idea that all workers should be organized into one big union. In the 1911 pamphlet One Big Union, IWW supporters Thomas J. Hagerty and William Trautmann enumerated two goals: One Big Union needed to "combine the wage-workers in such a way that it can most successfully fight the battles and protect the interests of the workers of today in their struggles for fewer hours of toil, more wages and better conditions," and it also "must offer a final solution of the labor problem – an emancipation from strikes, injunctions, bull-pens, and scabbing of one against the other."

One Big Union was the notional organizational concept, while the IWW's revolutionary industrial unionism was the organizing method by which that concept could be realized. "Organizing the One Big Union of all workers the world over" was meant to achieve "working class control". But the One Big Union organizations were resisted by government and industry, and subverted by existing trade unions. By 1925, only the slogan of One Big Union remained.^{[citation needed]}

The Industrial Workers of the World (IWW) adopted and promoted the concept of the One Big Union after the publication of the One Big Union pamphlet in 1911; the IWW continues to use the phrase. Members of the IWW historically, and currently, signed and sign letters (and other communications) with the closing, "Yours for the O.B.U." Many commentators regard One Big Union as synonymous with the Industrial Workers of the World. One of the popular IWW publications was called One Big Union Monthly.

The IWW promoted the One Big Union concept in various ways, including as an invitation to racial equality. One IWW leaflet proclaimed:

To Colored Workingmen and Women: If you are a wage worker you are welcome in the I.W.W. halls, no matter what your color. By this you may see that the I.W.W. is not a white man's union, not a black man's union, not a red or yellow man's union, but a working man's union. All of the working class in one big union.

The IWW used the same sort of arguments to welcome women into the workforce. The appeal subsequently proclaimed the intent to organize "all wage workers ... into One Big Union, regardless of creed, color, or nationality ... An injury to one is an injury to all." The One Big Union idea had the immediate goals of better pay, shorter hours, and better surroundings. The IWW propagandized, "Organize in one big union and fight for a chance to live as human beings should live. All together now and victory will be ours."